1. Field of the Invention
The present invention relates to a signal processing circuit for, for example, an optical disk apparatus which records information on an optical disk or reproduces information therefrom.
2. Description of the Related Art
Various optical disk apparatuses have been developed which cause a semiconductor laser oscillator (light source) in an optical head to irradiate a laser beam on an optical disk to form pits thereon, thereby recording data on the optical disk, or to convert data recorded on the optical disk into an electric signal by means of a detector (photodiode) provided in the optical head and then convert the signal a video signal (data signal) before reading it out.
In such an optical disk apparatus, an operational amplifier converts a data signal (current signal) picked up by a photodiode into a voltage signal. The voltage signal from this operational amplifier is processed by a signal processing circuit.
The amplitude of the data signal may become smaller relative to that of a large-amplitude signal originated from minute dust deposited on the disk and/or scratches thereon, thus greatly altering the level of a signal at a pit's edge portion. To digitize such a signal with high accuracy, the signal processing circuit performs the following operation.
The signal processing circuit detects the peak of the signal at the pit's edge portion, and feeds the detected peak back to the input side to adjust the pit edge level of the data signal output from the signal processing circuit. At this time an automatic bias controller is used to make the level of the output signal uniform. The data signal from the signal processing circuit is digitized to be demodulated for reproduction of the recorded data.
There is, however, a gap of several bits between the header portion (address portion) and the data portion in the signal read out from the disk. Unless the automatic bias controller of the above signal processing circuit has as quick a response as possible in the header portion, the head of the data (or the leading edge of the data stream) may not be read out due to an improper bias response.